Gas temperature and excitation classes in planetary nebulae

Empirical methods to estimate the elemental abundances in planetary nebulae usually use the temperatures derived from the [O III] and [N II] emission-line ratios, respectively, for the high- and low-ionization zones. However, for a large number of objects these values may not be available. In order to overcome this difficulty and allow a better determination of abundances, we discuss the relationship between these two temperatures. Although a correlation is not easily seen when a sample of different PNe types is used, the situation is improved when they are gathered into excitation classes. From [OII]/[OIII] and HeII/HeI line ratios, we define four excitation classes. Then, using standard photoionization models which fit most of the data, a linear relation between the two temperatures is obtained for each of the four excitation classes. The method is applied to several objects for which only one temperature can be obtained from the observed emission lines and is tested by recalculation of the radial abundance gradient of the Galaxy using a larger number of PNe. We verified that our previous gradient results, obtained with a smaller sample of planetary nebulae, are not changed, indicating that the temperature relation obtained from the photoionization models are a good approximation, and the corresponding statistical error decreases as expected.